1. Field of the Invention
This invention relates to a power supply device and an electric circuit, including a battery used as a power source for a load and having a function of easily monitoring whether an FET (field-effect transistor) for power-source output control, connected in series in a power output line from the battery, is operating normally or not (malfunction check function).
2. Description of the Related Art
A battery is used as a power source for driving loads such as robots and communication devices of various types. In a power supply device using a rechargeable battery such as an Ni-MH battery (nickel-hydride metal battery) or an Li ion battery, a switching device such as an FET is exclusively connected in series in a power output line from the battery. The switching device is brought into off-state, for example, when an overcurrent due to, for example, short-circuiting of a load is detected or output voltage is not sufficient. By the switching device coming into off-state, the rechargeable battery and the load are disconnected, so that the rechargeable battery is protected.
In the power supply device of this type, the FET (switching device) operating normally is a key to ensuring operational reliability of the power supply device. Hence, conventionally, as disclosed in Japanese Unexamined Patent Publication No. Hei 8-308143, a monitoring circuit for monitoring the source voltage of the FET is provided to detect malfunction due to formation of an open circuit between the gate and source of the FET (malfunction of the FET). When malfunction of the FET is detected, a warning is issued.
This malfunction check is carried out exclusively by making the power supply device operate with the load disconnected, and determining whether or not the power supply device is functioning normally in this state. Thus, the procedure is cumbersome, and malfunction check on the FET cannot be carried out in real time while the power supply device is being used actually. Further, malfunction of the FET is mostly due to thermal destruction, namely short-circuiting of the gate and source due to heat generated while the FET is operating. Hence, it is important to detect malfunction of the FET immediately while the power supply device is being used actually.